The present invention relates generally to a synthetic fiber cable, particularly as a support element for elevators, consisting of a bundle of reinforced load-bearing synthetic material fibers.
Reinforced synthetic fiber cables replace conventional wire cables to an increasing extent in applications such as, for example, with elevator installations where on the one hand large cable lengths are necessary and on the other hand for energy reasons the requirement exists for smallest possible moved masses.
Such synthetic fiber cables are a textile product of linear reinforced chemical fiber materials, preferably aramide or polyamide, which are spun into cable yarns and are produced by cable forming without rotation, by two-stage or multiple-stage twisting and/or sheathing. However, the cable breakage strength for chemical fibers significantly diminishes by comparison with steel cables even at substantially lower temperatures, before they finally melt. The melting point of aramides lies in the region of 450 to 500xc2x0 Celsius. At temperatures of above 180xc2x0 Celsius the load-bearing capability of synthetic fiber cables already begins to reduce.
In order to be able to use synthetic fiber cables of that kind, in particular as a running cable in conveying technology such as elevator construction, it is necessary to recognize the cable state free of doubt. For this purpose a device for recognition of readiness of synthetic fiber cables for discard is known from the European patent document 0 731 209 A1. The principle of function of this device is that indicator fibers are integrated in some of the strands of reinforced fibers with specific mechanical properties. The material properties of these indicator fibers depend on those of the load-bearing synthetic fibers, wherein the breaking elongation and the modulus of elasticity are so selected so that the indicator fibers break, due to material fatigue and abrasive wear, etc., earlier than the load-bearing fibers. These indicator fibers conduct current and are constantly monitored in terms of measurement technology. If the constant monitoring recognizes a predetermined number of failed indicator fibers, the cable has to be replaced by a new one.
With the device described so far with respect to construction and function, the breakdown of the cable caused by mechanical loading can be reliably recognized, but special fire protection requirements imposed on synthetic fiber cables cannot be satisfactorily fulfilled by that.
The present invention concerns a synthetic fiber cable provided with a temperature sensor to monitor the operational safety in the case of thermal overheating and/or in the case of fire. The synthetic fiber cable fulfils in particular the fire protection requirements in elevator construction where the safety of passengers must at no time be put at risk.
With the synthetic fiber cable according to the present invention a monitoring of the temperature in the cable and thus indirectly also over the entire shaft length and engine room is possible for the first time. The conductive connection by the cable is formed only at temperatures below the temperature critical for the cable. In the case of temperatures lying above that the conductive connection is interrupted and accordingly no electrical or optical signal or the like can be transmitted, which can be simply established in terms of measurement technology. In co-operation with a checking device, cable damage caused by heat can be detected promptly in this manner, reproduced at, for example, an elevator control, and appropriate measures for evacuation of passengers can be carried out by this without delay in time.
In a development of the invention, a temperature sensor element with temperature dependent conductivity for the applied checking signal is provided. This offers the advantage that starting out from a constructionally determined value, the strength of the checking signal can also be correspondingly changed. The respective cable temperature can be ascertained on the basis of this quantitative signal. The temperature-dependence can in that case be selected so that, on exceeding of the critical temperature, conductivity is no longer present.
A preferred development of the invention proposes that the temperature sensor element has a temperature-critical material strength that is lower than that of the load-carrying synthetic material fibers. On attainment of a constructionally predetermined temperature, the temperature sensor element fails in that, for example, it melts or breaks and thus interrupts the conductive connection. A qualitative checking signal, for the evaluation of which a very simple measurement technology is sufficient, is thereby obtained.
In preferred embodiments the temperature sensor element can also be constructed as electrical conductors, optical conductors or the like, by which a checking signal can be transmitted. Essential in the selection of the conductor material used in that case is a fatigue bending strength that at least corresponds with that of the load-bearing fibers, so that a work-induced material failure is excluded. For example, the temperature sensor element can be worked in with the cable as an electrical conductor in the form of a metal wire or a synthetic yarn or a material combination consisting thereof.
The temperature sensor element is preferably wound around the cable and covered by cable sheathing preferably formed in a pressure injection-molding press. In an advantageous embodiment in that case, several temperature sensor elements are arranged parallel to the strands and/or embedded, in the cable longitudinal direction, in the cable sheathing around the cable. This offers the advantage that the temperature sensor element can be laid closely against the cable structure and the mechanical loading of the temperature sensor element when running over rollers is small.